C1q, the recognition unit of the classical complement pathway, is a multifunctional glycoprotein consisting of collagen-like and globular domains. Evidence is accumulating to suggest that many cells including platelets and endothelial cells possess recognition sites for both the collagen-like region of C1q and for the globular domain. Because C1q accumulates at sites of vascular injury and inflammation, and has been identified in atherosclerotic lesions, platelet and endothelial cell C1q receptors may play an important role in the cellular response of blood to injury. To better understand the structure and function of platelet and endothelial cell C1qR, the present application will 1) investigate the cell surface expression and distribution of both cC1qR and gC1qR using immunochemical techniques and confocal scanning laser microscopy, 2) define ligand binding to cC1qR and gC1qR using purified c1, C1q, or its globular and collagen-like domains as well as monoclonal and polyclonal anti cC1qR and anti gC1qR antibodies, 3) identify gC1qR sequences responsible for ligand binding using site directed mutagenesis, 4) characterize the cellular response to C1qR occupancy using scanning electron microscopy to evaluate cell adhesion to immobilized C1q and extracellular matrix proteins, and biochemical and functional assays to assess platelet and endothelial cell activation, as well as C1q-induced heterotypic interactions between platelets, endothelial cells, monocytes, and neutrophils, and 5) evaluate the biosynthesis of endothelial cell cC1qR and gC1qR particularly in response to C1qR occupancy and inflammatory cytokines using biosynthetic and immunochemical assays. Results from these studies will provide new insights into the contribution of platelet and endothelial cell C1q receptors to vascular lesions and inflammation.